Currently, a wireless mobile communications system having fixed base stations, such as PHS (personal handyphone system®), has been in common use. Such a wireless mobile communications system realizes a function of detecting a position of a mobile terminal by referring to a positional information database of a fixed base station, taking advantage of the feature of a small wireless zone. Examples of a document disclosing this wireless mobile communications system include patent document 1 (Japanese Laid-Open Patent Application No. 9-247737/1997 (Tokukaihei 9-247737; published on Sep. 19, 1997)).
FIG. 12 shows a positional information detection system described in the above document. This positional information detection system includes: a mobile terminal 101, base stations 102a through 102d, respective wireless zones 103a through 103d for the base stations 102a through 102d, a general call area 104, a control station 105, respective telecommunications line connections 106a through 106d between the control station 105 and the base stations 102a through 102d, and a position management station 109. The base stations 102a through 102d register a calling area and a paging area with respect to the mobile terminal 101. The general calling area 104 consists of a plurality of wireless zones 103a through 103d. Sets of base station identification information 107a through 107d are issued respectively from the base stations 102a through 102d. The control station 105 controls line connections of the base stations 102a through 102d, with respect to the mobile terminal 101, respectively in the wireless zones 103a through 103d and is connected with a position management station 109 via a line 108.
Upon receipt of the sets of base station identification information 107a through 107d transmitted periodically from the base stations 102a through 102d, the mobile terminal 101 causes a control section to store positional information in a memory. The positional information is combined data of a reception electrical field value greater than a predetermined threshold value, which is measured by a reception electrical field strength measurement section, and base station identification information 107a, 107b, 107c, or 107d corresponding to the reception electrical filed value. Then, when a required number of sets of positional information to determine the position of the mobile terminal 101 is collected, the mobile terminal 101 performs transmission to a base station 102 (any one of 102a, 102b, 102c and 102d). A signal for positional information transmitted from the mobile terminal 101 is received by any of the base stations 102, and the base station 102 having received the signal transmits the signal to the position management station 109 via the control station 105. The position management station 109 refers to a database in accordance with the positional information acquired by demodulating the signal transmitted so as to determine a position of the mobile terminal 101.
In this manner, the wireless mobile communications system having fixed base stations can determine a current position of a mobile terminal by analyzing positional information data from a plurality of base stations in accordance with a reception electric strength information.
Meanwhile, a wireless mobile communications system free of fixed base stations, such as a specified low-power radio system disclosed in patent document 2 (Japanese Laid-Open Patent Application No. 2002-7525 (Tokukai 2002-7525; published on Jan. 11, 2002)), IrDA as disclosed in patent document 3 (U.S. Pat. No. 5,668,977 (published on Sep. 16, 1997)), IEEE802.11a as disclosed in patent document 4 (Japanese Laid-Open Patent Application No. 2002-57657 (Tokukai 2002-57657 (published on Feb. 22, 2002)), IEEE802.11b as disclosed in patent document 5 (Japanese Laid-Open Patent Application No. 2001-331429 (Tokukai 2001-331429 (published on Nov. 30, 2001)), and Bluetooth® as disclosed in patent document 6 (Japanese Laid-Open Patent Application No. 2002-7242 (Tokukai 2002-7242; published on Jan. 11, 2002)), are demanded to detect a position of a mobile terminal.
However, such a wireless mobile communications system free of fixed base stations cannot determine a relative distance between mobile terminals. This would cause a problem such as connection with a far, unintended terminal having a high transmission level regardless of the presence of a close terminal to be connected, misconnection with an unintended terminal because of the presence of a plurality of unintended terminals having the same transmission level as that of an intended terminal, and interceptions of communications data due to an unintended connection by a third party.